1. Field of the Invention
The present invention relates to a COD (chemical oxygen demand) measuring method which forms a COD conversion equation (regression equation) used for converting a ultraviolet-ray (UV) value to a COD value on the basis of the correlation between UV absorbance and a COD measured value in drainage, environmental water and the like, and based upon this conversion equation, provides a converted COD value from a UV value measured in sample water, and also concerns a device for such a method.
Here, COD is treated as a factor that includes BOD (biological oxygen demand).
2. Description of the Related Art
The Japanese Industrial Standard (JIS) has stipulated the ultraviolet photometer for monitoring of water pollution for use in water-quality monitoring, and the UV measured value is used in correlation with the COD value for calculating a water pollution loading amount and the like in connection with the total pollutant load control (see JIS K0807).
Upon measuring COD, an absorption photometer as shown in FIG. 6 is used. Light emitted from a light source 1 such as a low-pressure mercury lamp is directed to a measuring cell 2. The measuring cell 2 sends a sample to a space having a predetermined optical path length, and the light from the light source 1 is transmitted through the space. The light transmitted through the measuring cell 2 is allowed to transmit an interference filter 3 so that a specific wavelength, for example, 254 nm, is selected and detected by a detector 4, and the detection signal is converted to an absorbance value by an amplifier 5.
The COD component is mainly composed of organic substances, and most of the substances absorb ultraviolet rays; thus, by utilizing this characteristic, the correlation between the absorbance of a specific wavelength (normally 254 nm) in the ultraviolet region and a COD measured value obtained in a separate manner is examined to prepare a COD conversion equation so that the absorbance is converted to the corresponding COD value.
Moreover, a method has been proposed in which, by utilizing the fact that underwater turbidity mostly relates to visible light region, the absorbance of visible light (normally 546 nm) is measured simultaneously with the absorbance of ultraviolet rays so that influences from turbidity in sample water are removed by using the differential absorbance between the ultraviolet-ray absorbance and visible light absorbance.
With respect to the COD conversion equation, the following linear regression equation is used:Y=a+bX
(Y: converted COD value, X: UV absorbance value or differential absorbance value, a: intercept, b: gradient)
However, as shown in FIGS. 7 to 9, with respect to the organic compound and inorganic compound, the absorption spectrum in the ultraviolet region varies greatly depending on the substances. In the case of the normally-used 254 nm also, some exert a great absorbance value while others exert a small absorbance value, and some of them hardly exert any absorbance (see Table 1).
TABLE 1(Concentration 100 mg/L, Cell Length 10 mm)Name of compoundAbsorbanceName of compoundAbsorbancePhenol0.463Hippuric acid1.235p-Cresol0.396Albumin (bovine0.025serum)Phenyl acetate0.161Lignin (alkali)3.492Benzoic acid0.595Methanol0.000Cinnamic acid9.134Glucose0.000Salicyclic acid0.671L-Glutamic acid0.000Fumaric acid0.808Cellulose0.011Aniline0.771Acetic acid0.000(Extracted from JIS K 0807 Description)
Normally, various substances are contained in sample water, and the composition of substances varies depending on the sample water. For this reason, the COD conversion equation, obtained by using absorbance in a single ultraviolet-ray wavelength, tends to fail to provide a desirable correlation to COD.